* RHEL: [https://www.softwarecollections.org/en/scls/rhscl/python33/ Python 3.3] and [https://www.softwarecollections.org/en/scls/rhscl/rh-python34/ Python 3.4] are available on RHEL6 and RHEL7 using SCL.

* RHEL: [https://www.softwarecollections.org/en/scls/rhscl/python33/ Python 3.3] and [https://www.softwarecollections.org/en/scls/rhscl/rh-python34/ Python 3.4] are available on RHEL6 and RHEL7 using SCL.

* CentOS: [https://www.softwarecollections.org/en/scls/rhscl/python33/ Python 3.3] and [https://www.softwarecollections.org/en/scls/rhscl/rh-python34/ Python 3.4] are available on CentOS 6 and 7 using SCL (no Red Hat subscription needed, CentOS has its own flavor of SCL)

* CentOS: [https://www.softwarecollections.org/en/scls/rhscl/python33/ Python 3.3] and [https://www.softwarecollections.org/en/scls/rhscl/rh-python34/ Python 3.4] are available on CentOS 6 and 7 using SCL (no Red Hat subscription needed, CentOS has its own flavor of SCL)

Revision as of 12:14, 29 June 2016

This page tracks the progress of Python 3 effort porting for OpenStack.

IRC: #openstack-python3

Python 3

Python 3 is usually seen as the new Python version which breaks compatibility and raises new Unicode issues. Python 3 is much more than that. It’s a new clean language which has a more consistent syntax. It has many new features, not less than 15 new modules. Python 3 is already well supported by major Linux distributions, whereas Python 2.7 reached its end-of-life. Slowly, some bugs cannot be fixed in Python 2.7 anymore and are only fixed in the latest Python 3 release. Python 3 is now 5 years old and considered as a mature programming language.

Python 2: Python 2.6 support dropped, Python 2.7 only

OpenStack Liberty targets Python 2.7 and 3.4.

Python 2.6 support is being dropped in OpenStack since OpenStack Juno for servers. Python 2.6 support is currently kept in Oslo libraries and clients. See juno-cross-project-future-of-python etherpad.

Port Python 2 code to Python 3

OpenStack project chose to use the same code base for Python 2 and Python 3. The Six: Python 2 and 3 Compatibility Library helps to write code working on both versions. OpenStack supported Python 2.6 for RHEL up to Juno, but not Python 2.5 and older. As we are targeting Python 3.4 and up, there is no need to avoid u'unicode' syntax. Do not use six.u('unicode').

Before you begin

If you're doing development with Ubuntu/Debian (and not using devstack with the USE_PYTHON flag set), then you'll need the following packages installed to run the py34 tox unit test targets in the projects:

sudo apt-get install python3.4 python3.4-dev

sixer tool

The sixer tool helps to replace most basic patterns to add Python 3 compatibility and it respects OpenStack coding style.

Common patterns

Replace "for key in dict.iterkeys()" with "for key in dict"

Replace dict.iteritems() with dict.items()

Replace dict.itervalues() with dict.values()

Note: Replacing dict.iteritems()/.itervalues() with six.iteritems(dict)/six.itervalues(dict) was preferred in the past, but there was a discussion suggesting to avoid six for this. The overhead of creating a temporary list on Python 2 is negligible.

Replace json.loads(obj) with oslo_serialization.jsoutils.loads(obj): it accepts bytes and Unicode, bytes is decoded from UTF-8. It avoids "if isinstance(obj, bytes): obj = obj.decode('utf-8')" which may require a second temporary variable.

By default, the encoder and the decoder are strict. You can specify a different error handler using the optional errors parameter. Example: safe_encode(b'[\xff]', incoming='ascii', errors='ignore') returns b'[]'.

logging module and format exceptions

The exception_to_unicode(exc) function of oslo_utils.encodeutils is the recommanded way to format an exception to Unicode. This function works on Python 2 and Python 3 and it should avoid mojibake is most cases.

Common pitfalls

What is a string ?

You should definitely not talk about "strings" in your commit logs/reviews. In Python 2, a 'string' is bytes; in Python 3, it's a Unicode text string. The following code snippet may help in understanding the difference:

tox/testr error: db type could not be determined

The "db type could not be determined" error comes from .testrepository/times.dbm used by testr.

Workaround: "rm -rf .testrepository/" and then run "tox -e py34" before running "tox -e py27". You only have to do this once. The problem only occurs with "tox -e py34" when .testrepository/ was created by Python 2.

Sirushti Murugesan wrote and implemented the spec Python34 Support (which was accepted for Liberty). Mitaka version will fully support Python 3. Great job Sirushti Murugesan who did almost all the work!

Status at 2016-06-28: Matt Riedemann: "Keep in mind that Thursday 6/30 is the nova non-priority blueprint feature freeze (end of day really). So the majority of review focus this week should be on non-priority blueprints." Sylvain Bauzas: "Well, IIRC we discussed in the previous year on some of those blueprints (including the Py3 effort) that are not really features (rather refactoring items) and which shouldn't be hit by the non-priority feature freeze."

CentOS: Python 3.3 and Python 3.4 are available on CentOS 6 and 7 using SCL (no Red Hat subscription needed, CentOS has its own flavor of SCL)

In Debian, the plan is to deprecate Python 2 in Stretch (aka: Debian 9, the next Stable Debian after Jessie) and to completely remove Python 2 for the Buster release of Debian (aka: Debian 10, to be release in approximatively 2019).